Pressure effects on magnetism in Ca2Mn2O5 -type ferrites and manganites

The presence of ordered oxygen vacancies in perovskites governs magnetic phase stability owing to changes in crystal-field splitting with different anion geometries, polyhedral arrangements, and electronic configurations of the transition-metal cations. Here we use density functional theory calculations to assess the magnetic phase stability of ${\mathrm{Sr}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$ (with a $d^5$ electronic configuration) and ${\mathrm{Sr}}_2{\mathrm{Mn}}_2{\mathrm{O}}_5$ ($d^4$ configuration), exhibiting the ${\mathrm{Ca}}_2{\mathrm{Mn}}_2{\mathrm{O}}_5$-type oxygen-deficient perovskite structure, with hydrostatic pressure. The ${\mathrm{Ca}}_2{\mathrm{Mn}}_2{\mathrm{O}}_5$-type structure is composed of square pyramidal units, the crystal-field splitting and polyhedral connectivities of which support different ground-state magnetic orders depending on $d$-orbital filling: E-type antiferromagnetic (AFM-E) for ${\mathrm{Sr}}_2{\mathrm{Mn}}_2{\mathrm{O}}_5$ ($d^4$) and G-type antiferromagnetic (AFM-G) for ${\mathrm{Sr}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$ ($d^5$). We show that hydrostatic pressure enhances the crystal-field splitting and affects the magnetic stability. We find that the AFM-E order exhibited by ${\mathrm{Sr}}_2{\mathrm{Mn}}_2{\mathrm{O}}_5$ is robust over the surveyed ranges of applied pressures, whereas ${\mathrm{Sr}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$ shows a magnetic transition from AFM-G to ferromagnetic spin order at $\approx 24.5$ GPa. We also discuss the effect of correlation strength, treated using the Hubbard $U$ correction, which we find suppresses a spin crossover transition in ${\mathrm{Sr}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$ and shifts it to higher pressures.